Chapter 11

COLONIZATION-RELATED

TRADE-OFFSINTROPICAL

FORESTSANDTHEIRROLEIN

THEMAINTENANCEOFPLANT

SPECIESDIVERSITY

Helene C. Muller-Landau

OVERVIEW

Interspecific trade-offs involving colonization ability can contribute strongly to the maintenance of plant species
diversity, and are often cited as a potential mechanism underlying high tropical forest diversity. The well-known
competition–colonization trade-off, between the ability to win a regeneration site after arrival and the ability to
arrive, can in theory maintain very high species diversity, but only if there is strong competitive asymmetry among
species, such that the best competitor present is highly disproportionately likely to win. Other, less-studied trade-offs
involving colonization ability can contribute to diversity maintenance given appropriate habitat heterogeneity, by
facilitating habitat niche partitioning. Specifically, a trade-off between fecundity and stress tolerance combined with
corresponding variation in stress among regeneration sites can lead to coexistence between more tolerant species able
to win high stress sites and more fecund species that are numerically more likely to win low stress sites. A trade-
off between fecundity and dispersal can similarly contribute to coexistence given spatial variation in the density of
suitable regeneration sites. Empirical studies of species trait relationships, current understanding of the asymmetry
of competitive interactions among seedlings, and results of a seed addition experiment all suggest that the classical
competition–colonization trade-off is not present among tropical trees, and thus does not contribute to their coexis-
tence. In contrast, trait relationships do provide evidence for the presence of a tolerance–fecundity trade-off mediated
by seed size, with small-seeded species having higher fecundity and lower stress tolerance than large-seeded species.
Evidence concerning the existence of a dispersal–fecundity trade-off is mixed and inconclusive.To further elucidate the
roles of these colonization-related trade-offs, and specifically to assess their contributions, if any, to species coexistence
in tropical forests, we need additional studies of how spatio-temporal variation in environmental conditions and seed
arrival contribute to regeneration success, in natural systems, field experiments, and/or models.

INTRODUCTION

Interspecific trade-offs involving species’ abilities
to reach or “colonize” regeneration sites can
play multiple roles in niche partitioning and
diversity maintenance among tropical forest tree

speciesandinothercommunities.Thebest-known

examples are competition–colonization trade-offs

between a species’ ability to reach sites with its

recruits and the per recruit ability to win sites at

which recruits arrive (Tilman and Pacala 1993).

There is a long history of theoretical work on